sum in SQL?

[James Rich <james@xxxxxxxxxxx>]

I need to make a quick report that prints out total sales for each
customer.  I think SQL can do this nicely.  I want to print out one line
per customer with the totals for that customer from the invoice file,
something like:

Customer Num    Name            Inv Tot         Total YTD
187        0    Customer One    2,582.50           8,882.00
152        0    Customer Two    8,336.00          49,419.35
104        0    Customer Three     75.00             175.00

I thinking some kind of join of the invoice and customer files.  But then
How do I get SQL to do the summing of the invoice file for me?  If
Customer One has three invoices that total up to 2,582.50 how do I get SQL
to make that total and then stick on the line without listing the invoices
individually?

James Rich